1. Field of the Invention
The present invention relates to a single crystal silicon carbide useful as an electronic material and to a process for preparation of the same. In particular, the present invention relates to a single crystal silicon carbide having preferred characteristics in fabricating a semiconductor device such as low crystal defect density or small strain in crystal lattice, and to a process for preparation of the same.
2. Background Art
Crystal growth methods for silicon carbide (SiC) employed heretofore can be classified into two methods; one is bulk growth using sublimation method and the other is forming a thin film on a substrate by taking advantage of epitaxial growth.
The bulk growth using sublimation method has enabled polycrystalline polymorphs of high temperature phases, i.e., hexagonal (6H, 4H, etc.) silicon carbide, and it has also realized a substrate of SiC itself. However, this method suffered numerous defects (micropipes) induced inside the crystals, and found difficulty in increasing the area of the substrate.
On the other hand, the use of epitaxial growth on a single crystal substrate advantageously improves the controllability of impurities and realizes substrates with increased area, and overcomes the problem found in the sublimation method by reducing the formation of micro-pipes. Yet, in the epitaxial growth method, however, there is frequently found a problem concerning an increase in stacking fault defects attributed to the difference in lattice constants between the substrate material and the silicon carbide film. In particular, the use of silicon as the substrate to form thereon a silicon carbide leads to a considerable generation of twins and anti phase boundaries (APB) inside the growth layer of silicon carbide, because there is a large lattice mismatch between silicon carbide and the underlying silicon that is generally used as the substrate. Hence, these defects lead to a silicon carbide with inferior characteristics when applied to an electronic element.
As a method for reducing planar defects within the silicon carbide film, there is proposed, for instance, a technique for reducing the planar defects that are present in films having a specific thickness or more, said method comprising a step of forming a growth region on the substrate on which silicon carbide is grown, and a step of growing single crystal silicon carbide on the thus obtained growth region in such a manner that the thickness thereof should become equal to, or greater than, the thickness specific to the crystallographic direction on the growth plane (see JP-B-Hei6-41400, wherein the term xe2x80x9cJP-B-xe2x80x9d as referred herein signifies xe2x80x9can examined published Japanese Patent Applicationxe2x80x9d). However, since two types of anti phase boundaries that are formed inside silicon carbide tend to be extended with increasing film thickness in directions orthogonal to each other, the anti phase boundaries cannot effectively be reduced. Furthermore, the superstructure that is formed on the surface of the grown silicon carbide cannot be controlled as desired. Thus, if independently grown regions combine with each other, an anti phase boundary that newly generates at the bonded portion unadvantageously impairs the electric characteristics.
As a means for effectively reducing the anti phase boundary, K. Shibahara et al. proposed a growth method of silicon carbide onto an Si (001) surface substrate in which a surface axis normal to the Si (001) plane was slightly tilted from the crystallographic  less than 001 greater than  direction to the crystallographic  less than 110 greater than  direction (i.e., an offset angle was introduced) (see Applied Physics Letter, vol. 50 (1987) pp.1888). In this method, steps in atomic level are introduced equi-spaced in one direction by slightly tilting the substrate. Thus, planar defects that are provided in parallel with the thus introduced steps are allowed to propagate, whereas the propagation of the planar defects that are present in a direction perpendicular to the steps (i.e., the direction crossing the steps) is effectively suppressed. Accordingly, as film thickness of the silicon carbide increases, among the two anti phase boundaries included in the film, an anti phase boundary extending in the direction parallel to the introduced steps extends in preference to the anti phase boundary extending in the orthogonal direction to the steps. Thus, the anti phase boundaries can be effectively reduced. However, as is shown in FIG. 1, this method induces the generation of undesirable anti phase boundary 1 and twins due to an increase in step density at the boundary between the silicon carbide and the silicon substrate. Hence, this method still suffers from a problem that the anti phase boundaries cannot be completely extinguished. In FIG. 1, numeral 1 represents an anti phase boundary that have generated at the steps of single atoms, numeral 2 denotes a conjunction of anti phase boundaries, numeral 3 denotes an anti phase boundary generated at the terrace on the surface of the silicon substrate, represents an offset angle, and xcfx86 is an angle (54.7xc2x0) making the Si (001) plane with the antiphase boundary. The antiphase boundary 3 generated at the terrace on the surface of the substrate diminishes at the conjunction 2 of anti phase boundaries. However, the anti phase boundaries 1 generated at the monoatomic steps on the silicon substrate remain as they are because they have no counterparts for association.
Furthermore, in case of forming silicon carbide on the surface of a silicon substrate, an internal stress generates inside the silicon carbide layer due to the difference in the coefficient of thermal expansion between silicon and silicon carbide, to a mismatch in lattice constants, to the generation of defects that form inside silicon carbide, or to the influence of strain. Then, warping or strain generates on the silicon carbide that is formed on the silicon substrate attributed to the internal stress that is generated inside the silicon carbide layer. Hence, such a silicon carbide is unfeasible for use as a material for producing semiconductor devices.
In the light of such circumstances, an object of the present invention is to provide a process for producing a silicon carbide in which the anti phase boundaries are effectively reduced, and a process for producing a silicon carbide, in which the warping and strain attributed to internal stress are reduced.
Furthermore, another object of the present invention is to provide a single crystal silicon carbide reduced in anti phase boundaries and/or in warping and strain attributed to internal stress, and to a process for producing the same.
The aforementioned objects can be achieved by the present invention as follows.
In accordance with a first aspect of the present invention, there is provided a process for preparation of silicon carbide by depositing silicon carbide on at least a part of a surface of a substrate having on its surface undulations extending approximately in parallel with each other, wherein a center line average of said undulations is in a range of from 3 to 1,000 nm, gradients of inclined planes of said undulations are in a range of from 1xc2x0 to 54.7xc2x0, and, in a cross section orthogonal to a direction along which the undulations are extended, portions at which neighboring inclined planes are brought in contact with each other are in a curve shape.
In accordance with a second aspect of the present invention, there is provided a process for preparation of silicon carbide by depositing silicon carbide on at least a part of a surface of a substrate having on its surface undulations extending approximately in parallel with each other, wherein a center line average of said undulations is in a range of from 3 to 1,000 nm, gradients of inclined planes of said undulations are in a range of from 1xc2x0 to 54.7xc2x0, and said substrate is made of silicon or silicon carbide having a surface with a plane normal axis of  less than 001 greater than  crystallographic direction and with an area of {001} planes equal to or less than 10% of the entire area of the surface of said substrate.
According to a third aspect of the present invention, there is provided a process for preparation of silicon carbide by depositing silicon carbide on at least a part of a surface of a substrate having on its surface undulations extending approximately in parallel with each other, wherein a center line average of said undulations is in a range of from 3 to 1,000 nm, gradients of inclined planes of said undulations are in a range of from 1xc2x0 to 54.7xc2x0, and said substrate is made of silicon or cubic silicon carbide having a surface with a plane normal axis of  less than 111 greater than  crystallographic direction and with an area of {111} planes equal to or less than 3% of the entire area of the surface of said substrate.
In accordance with a fourth aspect of the present invention, there is provided a process for preparation of silicon carbide by depositing silicon carbide on at least a part of a surface of a substrate having on its surface undulations extending approximately in parallel with each other, wherein a center line average of said undulations is in a range of from 3 to 1,000 nm, gradients of inclined planes of said undulations are in a range of from 1xc2x0 to 54.7xc2x0, and said substrate is made of hexagonal silicon carbide having a surface with a plane normal axis of  less than 1,1,xe2x88x922,0 greater than  crystallographic direction and with an area of {1,1,xe2x88x922,0} planes equal to or less than 10% of the entire area of the surface of said substrate.
According to a fifth aspect of the present invention, there is provided a process for preparation of silicon carbide by depositing silicon carbide on at least a part of a surface of a substrate having on its surface undulations extending approximately in parallel with each other, wherein a center line average of said undulations is in a range of from 3 to 1,000 nm, gradients of inclined planes of said undulations are in a range of from 1xc2x0 to 54.7xc2x0, and said substrate is made of hexagonal silicon carbide having a surface with a plane normal axis of  less than 0,0,0,1 greater than  crystallographic direction and with an area of {0,0,0,1} planes equal to or less than 3% of the entire area of the surface of said substrate.
In accordance with a sixth aspect of the present invention, there is provided a process for preparation according to one of the first to fifth aspects above, wherein said silicon carbide is deposited from a vapor phase or a liquid phase.
In accordance with a seventh aspect of the present invention, there is provided a process for preparation according to one of the second to fifth aspects above, wherein, in a cross section orthogonal to a direction along which the undulations are extended, portions at which neighboring inclined planes are brought in contact with each other are in a curve shape.
In accordance with an eighth aspect of the present invention, there is provided a single crystal silicon carbide having a planar defect density equal to or less than 1,000 defects/cm2.
According to a ninth aspect of the present invention, there is provided a single crystal silicon carbide having an internal stress equal to or less than 100 MPa.
According to a tenth aspect of the present invention, there is provided a single crystal silicon carbide having a planar defect density equal to or less 1,000 defects/cm2 and an internal stress equal to or less 100 MPa.
According to an eleventh aspect of the present invention, there is provided a single crystal silicon carbide having an etch pit density equal to or less than 10/cm2 and a twin density equal to or less than 4xc3x9710xe2x88x924% by volume.
According to a twelfth aspect of the present invention, there is provided a process for preparation of the single crystal silicon carbide of one of eight to eleventh aspects above in accordance with any of the processes for preparation of first to seventh aspects above, wherein silicon carbide is epitaxially grown so as to maintain the crystallinity of the surface of the substrate to obtain the single crystal silicon carbide.
First Aspect
According to a first aspect of the present invention, a substrate having a plurality of undulations extending approximately in parallel with each other on the surface thereof is used as the substrate. By thus using a substrate having a plurality of undulations on the surface thereof, the effect of introducing an offset angle as described by K. Shibahara et al. can be achieved at the each inclined surfaces of the undulations. Furthermore, by using a substrate having a plurality of undulations on the surface thereof, the planar defects that are incorporated in the silicon carbide deposited on the substrate can be ameliorated as to minimize the strain, and the internal stress of the silicon carbide can be thereby reduced.
The undulations as referred in the present invention do not strictly require that they have mathematically defined parallelism or mirror plane symmetry, but that they have such a morphology sufficient for effectively reducing or diminishing the anti phase boundary. In addition, the undulations of the present invention are not atomic steps but are in a macro size compared to the atomic steps. This is because the undulations have a center line average 3 nm at the minimum, as mentioned below in detailed.
The undulations as referred in the present invention are constituted with repeated groove and ridge portions and the ridge portion has slopes (tilted or inclined planes) with gradient with respect to the base surface ranging from 1xc2x0 to 54.7xc2x0. The slopes of neighboring ridge portions are facing each other through the groove portions. Preferably, the undulations of the substrate surface is formed so as to fall integrated gradient angles of the slope with respect to the base surface in substantially zero, wherein the integration is made through the whole surface.
In the present invention, a silicon carbide layer is successively formed on the whole substrate surface or a part of the substrate surface, provided that such a part has the above-mentioned undulations. Since the substrate surface of the present invention has the undulations with the shape mentioned above, anti phase boundaries generated and grown at steps on the slopes with the growth of silicon carbide would associate each other between the undulations and thereby the anti phase boundaries are effectively eliminated to yield a single crystal silicon carbide with less defects.
The undulations that are formed on the surface of the substrate have a center line average in a range of from 3 to 1,000 nm. The undulations with a center line average of less than 3 nm is insufficient, because an effective offset angle cannot be obtained, and this leads to defects generated at a high density. If the centerline roughness should exceed 1,000 nm, the probability of causing collision between planar defects and thereby extinguishing them becomes low, and the effect of the present invention cannot be achieved. Thus, the surface of the substrate should have a center line average of 3 nm or higher but 1,000 nm or lower. To achieve the effect of the present invention in a further effective manner, the center line average is preferably 10 nm or higher but 100 nm or lower.
The center line average (Ra) used herein is that defined in JISBO601-1982. The definition of Rain the JIS is as follows: in a case, a portion has a measured length of xe2x80x9cLxe2x80x9d and is picked up from a roughness curve to the centerline, supposing the centerline of the picked-up portion as axis X, the direction of lengthwise amplification as axis Y, and roughness curve as y=f(x), the center line average (Ra) is defined as the following equation represented in micrometer.
Ra=(1/L)∫10|f(x)|dx 
The unit of center line average defined in JIS B0601-1982 is micrometer but the unit of center line average used in this invention is nanometer (nm).
The center line average of the surface of the substrate is measured by using an atomic force microscope (AFM).
Furthermore, the angle of the tilted planes of the undulations extending on the surface of the substrate above falls in a range of 1xc2x0 or higher but not higher than 54.7xc2x0.
In the process according to the present invention, the effect of the invention is exhibited by accelerating the growth of silicon carbide in the vicinity of the steps formed in atomic level on the surface of the substrate. Thus, present invention is realized in case the tilt angle of (111) plane of the undulations falls at an angle of 54.7xc2x0 or lower, at which the entire surface of the tilted plane is completely covered by a single step. In case where the gradient is provided at a tilt angle of less than 1xc2x0, the step density on the tilted plane of the undulations becomes extremely low. Hence, the gradient of the tilted planes of the undulations should be set at an angle of 1xc2x0 or higher. From the view point of realizing the effect of the present invention in a further effective manner, the tilt angle of the tilted planes of the undulations is preferably 2xc2x0 or higher but not higher than 10xc2x0.
In the present invention, the term xe2x80x9cthe tilted plane of the undulationsxe2x80x9d refers to planes having any morphology, such a flat planes, curved planes, etc. Furthermore, in the present invention, the term xe2x80x9ctilt angle of the tilted planes of the undulationsxe2x80x9d signifies the tilt angle of the tilted angle substantially contributing to the effect of the present invention, and it refers to an average tilt angle of the tilted planes. The tern xe2x80x9caverage tilt anglexe2x80x9d signifies the angle that is made by the plane along the crystallographic orientation and the tilted plane (an average value for the evaluated region).
Further, in a cross section orthogonal to a orientation along which the undulations are extended, portions at which neighboring inclined planes are brought in contact with each other are in a curve shape. The portion at which the neighboring tilted planes are brought into contact with each other refers to the groove portion and the ridge portion of the undulations extending on the surface of the substrate, and the bottom of the groove as well as the apex of the ridge is provided in a curved shape. This state can be understood from the AFM image shown in FIG. 4. More specifically, the cross section of the undulations as observed on the cross section orthogonal to the orientation along which the undulations are extended exhibits a shape similar to a sinusoidal wave, although the wavelength and the height of the wave not necessarily yield constant values. Thus, the density of planar defects can be reduced by thus providing bottom portions of the groove and the apex of the ridge with curved cross sections.
Thus, by providing a plurality of undulations on the surface of the substrate on which silicon carbide is grown as described above, the effect of introducing offset angles as shown by K. Shibahara et al. can be realized at each of the tilted planes of each of the undulations. From the viewpoint of the limits of the etching technology in forming undulations on the substrate, the interval between the apices of the undulations is preferably 0.01 xcexcm or larger. On the other hand, since the frequency of causing association of the anti phase boundaries greatly decreases in case the interval between the apices of the undulations exceeds 1,000 xcexcm, the interval between the apices of the undulations is preferably 1,000 xcexcm or smaller. Furthermore, from the viewpoint of sufficiently exhibiting the effect of the present invention, the preferred interval between the apices of the undulations is 0.1 xcexcm or larger, but not more than 100 xcexcm.
The height difference and the interval between the undulations affect the gradient of the undulations, i.e., the step density. Since the preferred step density depends on the conditions of crystal growth, it cannot be limited to a specific value, however, in general, the height difference of the undulations is approximately the same as the interval between the apices of the undulations; that is, the height is preferably 0.01 xcexcm or larger, but not more than 20 xcexcm.
In accordance with the first aspect of the present invention, usable as the materials for the substrate are, for instance, single crystal substrates such as those of silicon or silicon carbide, or of sapphire.
Further according to the first aspect of the present invention, silicon carbide is deposited on at least a part of the surface of the substrate.
Those described above are also common to other aspects of the present invention that are described hereinafter.
Second Aspect
Similar to the first aspect described above, in the second aspect according to the present invention, used as the substrate for depositing thereon silicon carbide is a substrate having a plurality of undulations extended approximately in parallel with each other on the surface thereof, said undulations having a center line average in a range of from 3 to 1,000 nm, and having tilted planes having a tilt angle in a range of from 1xc2x0 to 54.7xc2x0. The reasons for limiting the center line average of the undulations above to a certain range, for setting a preferred range thereof; or the reasons for limiting the tilt angle of the tilted planes of the undulations, for setting a preferred range thereof; as well as the other points that are set in common, are the same as those described above in the first aspect of the present invention. In the second aspect of the invention, however, the substrate used therein is a silicon or a silicon carbide substrate, and the surface thereof has crystallographic orientation as such that its plane normal axis is set along the  less than 001 greater than  crystallographic orientation, with the {001} planes accounting for 10% or less of the entire area of the surface of the substrate. By thus providing undulations on the surface of the substrate and by thus controlling the ratio of the planar planes remaining on the surface of the substrate in this manner, the internal stress of the silicon carbide that is formed by deposition on the substrate can be controlled.
A crystallographic (001) plane generates a tensile stress with respect to the film as it grows in the crystallographic  less than 001 greater than  direction. However, by forming undulations on the surface of the substrate and by thus increasing the ratio of (111) planes, a compression stress, which cancels out the tensile stress of the (001) plane, can be intentionally generated as to relax the in-plane stress. For instance, by using an undulated substrate in which the ratio of the (001) planes is controlled as such to account for 10% or less of the entire surface of the substrate, and in which the undulations are formed as such that they may have tilted planes inclusive of (111) planes and the like in such a manner that the growing crystalline phases may collide with each other, a tensile stress generates in the crystallographic  less than 001 greater than  direction while a compression stress generates in a direction perpendicular to the  less than 001 greater than  plane as to cancel out each other. In this manner, the stress can be controlled. Ideally, the {001} planes account, in the lower end, for 0% of the area of the entire surface of the substrate.
Third Aspect
Similar to the first aspect of the present invention described above, in the third aspect of the present invention, used as the substrate for depositing thereon silicon carbide is a substrate having a plurality of undulations extended approximately in parallel with each other on the surface thereof, said undulations having a center line average in a range of from 3 to 1,000 nm, and having tilted planes having a tilt angle in a range of from 1xc2x0 to 54.7xc2x0. The reasons for limiting the center line average of the undulations above to a certain range, for setting a preferred range thereof; or the reasons for limiting the tilt angle of the tilted planes of the undulations, for setting a preferred range thereof; as well as the other points that are set in common, are the same as those described above in the first aspect of the present invention. In the third aspect of the invention, however, the substrate used therein is a silicon or a cubic silicon carbide substrate, and the surface thereof has crystallographic orientation as such that its plane normal axis is set along the  less than 111 greater than  crystallographic orientation, with the {111} planes accounting for less than 3% of the entire area of the surface of the substrate. By thus providing undulations on the surface of the substrate and by thus controlling the ratio of the planar planes remaining on the surface of the substrate in this manner, the internal stress of the silicon carbide that is formed by deposition on the substrate can be controlled in a manner similar to the case as described in the second aspect above. Ideally, the {111} planes account, in the lower end, for 0% of the area of the entire surface of the substrate.
Fourth Aspect
Similar to the first aspect of the present invention described above, in the fourth aspect of the present invention, used as the substrate for depositing thereon silicon carbide is a substrate having a plurality of undulations extended approximately in parallel with each other on the surface thereof, said undulations having a center line average in a range of from 3 to 1,000 nm, and having tilted planes having a tilt angle in a range of from 1xc2x0 to 54.7xc2x0. The reasons for limiting the center line average of the undulations above to a certain range, for setting a preferred range thereof; or the reasons for limiting the tilt angle of the tilted planes of the undulations, for setting a preferred range thereof; as well as the other points that are set in common, are the same as those described above in the first aspect of the present invention. In the fourth aspect of the invention, however, the substrate used therein is a hexagonal silicon carbide substrate, and the surface thereof has crystallographic orientation as such that its plane normal axis is set along the  less than 1,1,xe2x88x922,0 greater than  crystallographic direction, with the {1,1,xe2x88x922,0} planes accounting for 10% or less of the entire area of the surface of the substrate. By thus providing undulations on the surface of the substrate and by thus controlling the ratio of the planar planes remaining on the surface of the substrate in this manner, the internal stress of the silicon carbide that is formed by deposition on the substrate can be controlled in a manner similar to the case as described in the second aspect above. Ideally, the {1,1,xe2x88x922,0} planes account, in the lower end, for 0% of the area of the entire surface of the substrate.
Fifth Aspect
Similar to the first aspect of the present invention described above, in the fifth aspect of the present invention, used as the substrate for depositing thereon silicon carbide is a substrate having a plurality of undulations extended approximately in parallel with each other on the surface thereof, said undulations having a centerline roughness in a range of from 3 to 1,000 nm, and having tilted planes having a tilt angle in a range of from 1xc2x0 to 54.7xc2x0. The reasons for limiting the center line average of the undulations above to a certain range, for setting a preferred range thereof; or the reasons for limiting the tilt angle of the tilted planes of the undulations, for setting a preferred range thereof; as well as the other points that are set in common, are the same as those described above in the first aspect of the present invention. In the fifth aspect of the invention, however, the substrate used therein is a hexagonal silicon carbide substrate, and the surface thereof has crystallographic orientation as such that its plane normal axis is set along the  less than 0,0,0,1 greater than  crystallographic direction, with the {0,0,0,1} planes accounting for 3% or less of the entire area of the surface of the substrate. By thus providing undulations on the surface of the substrate and by thus controlling the ratio of the planar planes remaining on the surface of the substrate in this manner, the internal stress of the silicon carbide that is formed by deposition on the substrate can be controlled in a manner similar to the case as described in the second aspect above. Ideally, the {0,0,0,1} planes account, in the lower end, for 0% of the area of the entire surface of the substrate.
Sixth Aspect
According to the sixth aspect of the present invention, silicon carbide is deposited on at least a part of the surface of the substrate from a vapor phase or a liquid phase. Any known methods for depositing silicon carbide from a vapor phase or a liquid phase can be used as they are for the deposition.
As the gaseous source material for depositing silicon carbide from a vapor phase, usable are the silane based gaseous compounds such as dichlorosilane (SiH2Cl2), SiH4, SiCl4, SiHCl3, etc. As the gaseous source material for carbon, usable are gaseous hydrocarbon such as acetylene (C2H2), CH4, C2H6, C3H8, etc.
As a liquid phase process, there can be mentioned a method comprising melting a polycrystalline or amorphous silicon carbide, or a method comprising producing silicon carbide from a silicon source and a carbon source.
Seventh Aspect
According to the seventh aspect of the present invention, there is provided, in the method for producing silicon carbide in accordance with any of the second to fifth aspects of the present invention, a method characterized by that, in a cross section orthogonal to an orientation along which the undulations are extended, portions at which neighboring inclined planes are brought in contact with each other are in a curve shape. The portion at which the neighboring inclined planes are brought in contact with each other refers to a portion corresponding to the grooves and the ridges of the undulations extended on the surface, and the bottom of the grooves as well as the apex of the ridges yield a curved cross section. This state can be understood from the AFM image shown in FIG. 4. More specifically, the cross sectional shape of the undulations need not yield a constant wavelength or wave height, but it yields some kind of a sinusoidal curve. The planar defect density can be reduced by providing undulations having curved cross sections at the bottom portion of the grooves and at the apex portion of the ridges.
The undulations having specific shapes as described above are formed on the surface of a substrate by means of, for instance, photolithography, press working, laser processing, ultrasonic process, polishing, etc. Any method can be employed so long as the surface of the substrate yields a final morphology as such to effectively reduce or extinguish the anti phase boundaries as described in the aspects of the present invention.
By using photolithograpy to form a desired mask pattern to be transferred, a desired undulated shape can be formed on the substrate. Furthermore, the width of the undulations can be controlled by changing, for instance, the line width of the pattern; whereas the depth and the angle of the tilted plane can be controlled by controlling the selectivity ratio of the resist and the substrate. In case of forming a substrate having a curved shape for the portion at which the tilted planes are brought into contact with each other in the cross section perpendicular to the orientation along which the undulations are extended on the surface of the substrate, an undulated pattern having a curved shape (wavy shape) can be obtained by transferring a pattern to the resist, and by then reflow of the resist by heat treatment.
By using press working, a desired undulated shape can be formed on the substrate by forming the pressing mold into the desired shape. By thus forming various shapes, a variety of undulated shapes can be formed on the substrate.
The use of laser process or ultrasonic processing enables a still finer etching of the substrate because the undulations are directly formed on the substrate.
By using polishing, the width and the depth of the undulations can be controlled by varying the size of the abrasive grains and the working pressure. In case of producing a substrate having provided thereon an undulated pattern along one direction, the polishing is applied only on one direction.
By employing a dry etching process, the width and the depth of the undulations can be controlled by varying the conditions of etching and the shape of the etching mask. In case of forming a substrate having a curved shape for the portion at which the tilted planes are brought into contact with each other in the cross section perpendicular to the orientation along which the undulations are extended on the surface of the substrate, a wave-like pattern having curved cross section can be obtained by aligning the etching mask at a distance from the substrate onto which the pattern is to be transferred, because etching proceeds diffused between the mask and the substrate. Otherwise, a mask having a mask window with a trapezoidal cross section extended to the side of the substrate on which the pattern is to be transferred.
Eighth Aspect
According to the eighth aspect, there is provided a single crystal silicon carbide having a planar defect density of 1,000/cm2 or lower. A single crystal silicon carbide is well known in the art, but the single crystal silicon carbide known heretofore had planar defects at a density exceeding 104/cm2 (see, for instance, A. L. Syrkin et al., Inst. Phys. Conf. Ser. No.142, p189). In contrast to the above, the single crystal silicon carbide according to the present invention has a planar defect density of 1,000/cm2 or lower, and preferably, 100/cm2 or lower. The lower limit of the planar defect density is ideally 0/cm2 but in practice, the planar defect is present at a density of about 0.1/cm2 Because such a single crystal silicon carbide has a small crystal boundary density, and exhibits extremely superior electric characteristics it can be suitably used as a semiconductor substrate, a substrate for growing thereon a crystal (inclusive of seed crystals), or for other types of electronic devices.
Ninth Aspect
In accordance with the ninth aspect of the present invention, there is provided a single crystal silicon carbide having an internal stress of 100 MPa or lower. A single crystal silicon carbide is well known in the art, but a polycrystalline silicon carbide known heretofore yields an internal stress exceeding 100 MPa (reference can be made to, for instance, T. Shoki et al., SPIE. Int. Soc. Opt. Eng. Vol. 3748, p456). In contrast to above, the single crystal silicon carbide according to the present invention yields an internal stress of 100 MPa or lower, and preferably, 50 MPa or lower. The lower limit of the internal stress is ideally 0 MPa, and in practice, is 50 MPa. Such a single crystal silicon carbide suffers less warping and strain, and enables a silicon carbide having a flat surface. In case silicon carbide is curved due to the internal stress, the surface of the silicon carbide suffers strain. For instance, in case of newly depositing silicon carbide on a substrate using the silicon carbide above, the silicon carbide thus deposited succeeds the strain of the underlying substrate. However, if a flat and strain-free single crystal silicon carbide having an internal stress of 100 MPa or lower according to the present invention is used as the substrate, the problem mentioned above can be circumvented.
Tenth Aspect
A tenth aspect according to the present invention provides a single crystal silicon carbide having a planar defect density of 1,000/cm2 or lower and an internal stress of 100 MPa or lower. As described above, a single crystal silicon carbide known heretofore suffers planar defects exceeding a density of 104/cm2, and no single crystal having an internal stress of about 100 MPa has been found to date. In contrast to above, the single crystal silicon carbide according to the present invention has a planar defect density of 1,000/cm2 or lower, and preferably, 100/cm2 or lower, with an internal stress of 100 MPa or lower, and preferably, 50 MPa or lower. The lower limit of the planar defect density is ideally 0/cm2, but in practice, the planar defect is present at a density of about 0.1/cm2. Further, the lower limit of the internal stress is ideally 0 MPa, but in practice, the internal stress is 50 MPa. Such a single crystal silicon carbide exhibits extremely superior electric characteristics attributed to a low crystal boundary density, and can be suitably used as a semiconductor substrate, a substrate for growing thereon a crystal (inclusive of seed crystals), or for other types of electronic devices. At the same time, it enables a flat silicon carbide having extremely low warping and strain.
Eleventh Aspect
In accordance with the eleventh aspect of the present invention, there is provided a single crystal silicon carbide having an etch pit density equal to or less than 10/cm2 and a twin density equal to or less than 4xc3x9710xe2x88x924% by volume. The etch pit density affects the production yield of devices using a single crystal silicon carbide of the present invention and 90 percent or more of production yield can be accomplished for 0.01 cm2 of a device area when the etch pit density is equal to or less than 10/cm2. It is preferred from the viewpoint of improved production yield that the etch pit density is equal to or less than 1/cm2. The twin density is advantageous to be equal to or less than 4xc3x9710xe2x88x924% by volume because 90 percent or more of production yield can be accomplished for 0.01 cm2 of a device area and the twin density is more advantageous to be equal to or less than 4xc3x9710xe2x88x925% by volume.
Twelfth Aspect
In accordance with the twelfth aspect of the present invention, there is provided a method for producing the single crystal silicon carbide as described in the eighth to the eleventh aspects above, wherein, in any of the methods for producing a silicon carbide as described in one of the first to the seventh aspects above, silicon carbide is epitaxially grown by succeeding the crystallinity of the surface of the substrate. Any method for epitaxially growing silicon carbide can be employed so long as it is capable of limiting the propagation of the planar defects within the film in a confined crystallographic orientation while succeeding the crystallinity of the surface of the substrate. Specifically mentioned methods are, for instance, chemical vapor deposition (CVD) method, liquid phase epitaxial (LPE) growth method, sputtering method, molecular beam epitaxy (MBE) method, etc. Furthermore, in the case of employing a CVD process, the gaseous source material may be supplied simultaneously instead of employing the alternating gas supply method.
In accordance with the twelfth aspect of the present invention, the steps are introduced to the surface of the substrate on which silicon carbide is grown in such a manner that they might yield a statistically balanced density in the planes having a mirror plane symmetry. Hence, a silicon carbide film completely devoid of anti phase boundaries can be obtained because the anti phase boundaries unexpectedly introduced inside the silicon carbide layer attributed to the presence of the steps within the surface of the substrate effectively undergo association with each other. Furthermore, by the effect of introducing the offset angles, the growth regions all become regions with the same orientation. Thus, advantageously, even in case the discrete growth regions combine with each other as they grow, anti phase boundaries do not generate at the combined portions.
More specifically, in accordance with this method, the mismatch in lattice constant at the boundary between silicon and silicon carbide, which is known to be problematic on depositing a silicon carbide film on a silicon substrate, can be overcome while suppressing the generation of defects. Hence, the method enables the formation of a high quality silicon carbide.